The invention relates to the field of optical fiber telecommunications. In order to increase the capacity of optical fiber telecommunications systems, the current trend is to use a certain number of transmission channels simultaneously on slightly different wavelengths, using a scheme referred to as xe2x80x9cwavelength division multiplexingxe2x80x9d or xe2x80x9cWDMxe2x80x9d. Among the various components necessary for forming such a WDM system, optical multiplexers are essential to enable optical signals to be added to and/or dropped from the transmission line (optical fiber) at given wavelengths.
Such filters are known to the person skilled in the art as xe2x80x9coptical add-drop multiplexersxe2x80x9d or xe2x80x9cOADMsxe2x80x9d, this term being used in the description below.
An OADM may be formed by cascading two functions: i) wavelength selection (filter); and ii) coupling between the transmission line and an input/output branch thereof. The two functions can be performed by a single optical component (a fiber coupler with a Bragg grating in the coupling region) or by an assembly comprising a plurality of components (a Bragg filter, and an optical circulator, for example). In WDM systems with a large number of channels having narrow wavelength spacing, i.e. xe2x80x9cdense WDMxe2x80x9d or xe2x80x9cDWDMxe2x80x9d, the optical filters must be very narrow (xcx9c0.2 nm) with steep edges (xcexc0.1 nm).
A problem encountered when such filters are formed by inscribing a Bragg grating in an optical fiber is a high increase in chromatic dispersion at the borders of the working band of the filter. The excessive chromatic dispersion gives rise to unacceptable widening of the light pulses and penalizes transmission performance, thereby imposing a lower limit for spacing between the WDM channels, and thus an upper limit on the number of channels that can be transmitted in a given bandwidth.
Document D1, G.Nykalak et al., xe2x80x9cImpact of fiber grating dispersion on WDM system performancexe2x80x9d, OFC ""98 Technical Digest, paper TuA3, pp. 4-5 (1988) demonstrates that problem experimentally, with the conclusion that chromatic dispersion at the borders of the bands of fiber Bragg grating (FBG) filters can be detrimental to the capacity of a DWDM system because it imposes a limit on the relationship between the width of the WDM channels and their spacing (spectrum use).
A partial solution to that band border problem is proposed in Document D2, M. Ibsen et al., xe2x80x9cOptimised square passband fibre Bragg grating filter with flat group-delay responsexe2x80x9d, Elec. Lett. 34 (8) pp. 800-802 (Apr. 16, 1998). According to the teaching of D2, the refractive index profile is modulated using a xe2x80x9csincxe2x80x9d (xxe2x88x921 sin x) function that is truncated (because it is of finite length). Gaussian apodization makes it possible to overcome xe2x80x9cGibbs"" phenomenonxe2x80x9d at least in part, i.e. an increase in the reflectivity of the Bragg grating at the borders of the band.
Other teaching of the prior art concerns the cascading of two non-linear Bragg filters for compensating high-order dispersion. In order to broaden the working band of dispersion compensators, in particular third-order dispersion compensators, Document D3, T. Komukai et al., OFC ""98 Technical Digest, pp. 71-72, session TuM2, (1998), proposes to concatenate Bragg filters with inverted non-linear xe2x80x9cchirpsxe2x80x9d. The word xe2x80x9cchirpxe2x80x9d is used by the person skilled in the art to designate a monotonic variation in group delay with varying wavelength. It can be obtained by varying the pitch of the Bragg grating along the filter. Cascading filters with inverted chirps means that the group delay increases with increasing wavelength in one of the filters, and decreases with increasing wavelength in the other filter.
The two solutions proposed in Documents D2 and D3 remain difficult to implement industrially, hence the need to find a solution that is easier to implement.
An object of the present invention is to mitigate the problems presented by the solutions known from the prior art, either as regards performance obtained, or as regards the ease with which industrial implementation is possible.
To these ends, the invention provides a Bragg grating optical filter device having a broad working band, and having a group delay that is flat in the working band, as defined in claim 1. According to claim 1, there are two Bragg gratings in series, each of which has approximately linear chirp that is inverted relative to the chirp of the other filter along the length of the filter.
In a particular embodiment, the two Bragg filters are connected successively between an input port and output port by means of a four-port circulator.
In another embodiment, the two Bragg filters are connected between the input port and the output port by two three-port optical circulators connected together by a light guide.